The present invention relates to an inspection system in an automated production line. More particularly, the invention relates to a system and method of inspection of ophthalmic lens blisters after they are thermally sealed.
Ophthalmic lenses are packaged in small containers commonly called as blister packs. The containers typically contain a single ophthalmic lens submerged in a solution. Typically a printed foil made of aluminum is applied with a heat seal coating of the Vinyl type, or coatings based on cellulose nitrate, ethyl celluloses, methacrylates, Chlorinated rubbers or rubber polymers depending upon the application. Such a foil is applied to the top of such containers and sealed thereto to form an airtight seal that prevents the solution from leaking out of the container and thereby exposing the ophthalmic lens to contamination. The sealing quality is critical as it determines the quality of the lens. A good seal protects the lens therein from getting contaminated. A faulty seal can expose the solution and the lens to oxidation and result in the entire product to be discarded due to contamination. The inspection system and the related software inspects the captured images in and around the sealing area of the pocket and makes a decision of the sealing quality by applying several types of algorithms to check the uniformity of the seal in comparison with a predetermined pattern. The pattern may include but not limited to gray scale levels of a batch of pixels in three different color planes and width of the sealing pattern in different color regions. The difference in the calculated values compared with a predetermined set of values is then compared with the acceptable values stored in memory during configuration setup for a particular model of the product. The comparison is repeated over the entire perimeter of the seal. If any value falls outside the preset range, the product will be classified as a reject. If all values fall within the preset range, the product will be classified as a pass.
For a perfectly good seal, the thermal heat and pressure applied to the sealing foil and the time for which it is applied at the correct position has to be optimal and consistent. If the process of sealing is controlled to maintain the parameters from blister to blister, the range of gray level values at the thermally sealed areas under multiple illumination types will be consistently uniform for a good seal.
The sealing is performed around the perimeter of the container using an automated mechanism that have heated blocks of metal the shape of which is customized to seal only the periphery of the blister socket. The heated block is pressed against the top side of the foil at a predetermined pressure, to form a bond between the foil and the plastic foil and the container therein. Even though the sealing mechanism is automated, problems occur during the sealing process. (For Eg.): Improper positioning of the plastic container may result in the sealing being offset, insufficient temperature of the heated blocks of metal that result in a weak seal . . . etc. These deficiencies in the process results in an imperfect seal that will result in leaking of the saline solution leading to contamination and other forms of damage to the lens. Such defective blister packs are then shipped to the customer. It is therefore desirable to inspect the sealing quality, to ensure that imperfectly sealed blisters are detected and removed to enable good quality product is delivered to the customers.
Current methods of inspection include manual inspection of statistically sampled blisters at different process points, Manual inspection using UV light, testing in vacuum enclosures and manual inspection of back lit blisters. Manual inspection is highly prone to human mistakes and not effective especially for mass production.
In one form of inspection, infrared Cameras are used to check the sealing quality by analyzing the thermal images captured immediately after thermal sealing process. This method requires accurate timing for image capture to maintain consistency of thermal characteristics from package to package. A drawback encountered in this system and method of inspection, is the need to have consistent blister package quality and precise control over temperature of the sealing pads to ensure stability in image quality. Moreover some of the defects cannot be enhanced in the infrared region of illumination. There is a high probability that defective seals can be passed off as good seal.
Prior art discloses a system for inspecting contact blister package utilizing a camera that captures images of the bottom side of the blister. Such images are not suitable to perform a proper inspection of the seal which is present at the opposite side of the blister. This method and apparatus suffers from a drawback in which the blister pack polymer material prevents or masks important features of the sealing area leading to false rejects or in worse cases faulty seal blisters being shipped to customer.
This imaging technique is able to inspect foreign particles within the sealed area, but it is not effective for inspecting the sealing quality. Most of the sealing related defects have very low contrast in such configuration because of the presence of the semi-transparent blister polymer material.
Prior art also discloses the use of Thermal imaging to inspect the quality of the sealing. Thermal imaging techniques are unable to detect very fine defects such as gaps and bubbles within the sealing area.
Further, in prior art systems, images are captured under a single illumination typically with a dedicated wavelength. The resulting image is compromised as many types of defects are not highlighted. The image is therefore is unreliable for inspection resulting in defective products shipped to the customer.
It is well known that manual inspection is neither reliable nor efficient.
It is therefore desirable to have a robust system and method to accurately inspect the sealing quality of the blister packs from the top side where the thermal seal of the blister is visible. Images captured in this configuration enables the system to detect multiple defects such as air gaps in the sealing area, missing seal, imperfect seal and contaminated particles within the seal, so as to reject and remove imperfectly sealed blister packs and ensure good quality product is shipped to the customer.